Changing default text value in file dialogue
It's actually a feature of the ExportHelper mixin:

class ExportHelper:
    filepath = StringProperty(
            name="File Path",
            description="Filepath used for exporting the file",
            maxlen=1024,
            subtype='FILE_PATH',
            )
    #...
    # subclasses can override with decorator
    # True == use ext, False == no ext, None == do nothing.
    check_extension = True

    def invoke(self, context, event):
        import os
        if not self.filepath:
            blend_filepath = context.blend_data.filepath
            if not blend_filepath:
                blend_filepath = "untitled"
            else:
                blend_filepath = os.path.splitext(blend_filepath)[0]

            self.filepath = blend_filepath + self.filename_ext

        context.window_manager.fileselect_add(self)
        return {'RUNNING_MODAL'}

    def check(self, context):
        # code that uses check_extension and adds it if True
        # ...


Add this to your export operator to overwrite the invoke() method:

    def invoke(self, context, event):
        self.filepath = ""
        context.window_manager.fileselect_add(self)
        return {'RUNNING_MODAL'}


It won't use a default name for the export, clears the filepath from a previous run of the operator (so always starts empty), but still adds the file extension if you enter a file name (you can disable it if you want).